drm/display/dp: Add helper function to get DSC bpp precision

Add helper to get the DSC bits_per_pixel precision for the DP sink.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Reviewed-by: Sui Jingfeng <suijingfeng@loongson.cn>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231110101020.4067342-2-ankit.k.nautiyal@intel.com